Another confirmation of his mother's maiden name being Davies is from the new GRO index (registration needed but free)
MCLEAN, HARRY MORTIMER DAVIES
GRO Reference: 1872 M Quarter in MANCHESTER Volume 08D Page 321
(you could get an exact date of birth by ordering the certificate but from the date of registration, the 1871 date is possible, the 1873 date not so)
There is a possible marriage of Amelia Davies to an Anthony Mclean registered in W Derby in 2Q of 1863 MCLEAN Anthony W. Derby 8b 649
You mentioned Northallerton.
The 1871 census for Newton, Lancashire RG10; Piece: 4056; Folio: 88; Page: 37 has
Anthony Maclean age 35 Inland Revenue Officer Northallerton
Amelia,wife 29, Wales
William, son, 4
Charlotte A,dau 3
Charles L.,son 1 ( says all these children were born in Wales)
Anthony Mclean, by then supervisor in the Inland Revenue died on 26th Jan 1892 leaving his estate worth £352 10s, administration granted to wife Jane, (you can find this in the Calendar of Probate on Ancestry or https://www.gov.uk/search-will-probate
Anthony Mclean was buried at St Mary, Mellor, Lancashire on January 30th 1892, the burial register (ref PR 3093/1/4 ) says he was 56 years old and lived in St Pauls Parish Blackburn
Also buried at St Mary, Mellor, Lancashire(PR 3093/1/4 ) was Amelia Mclean.
She was buried on 21, Jan 1880, said to be 38 years old and also lived in St Pauls Parish Blackburn.
The marriage of Anthony and Amelia is seems to have taken place in Everton
Liverpool Records Office, Everton, St George, 283 GEV/3/5
It took place on May 28th 1863, at Everton, St George, Walton on the Hill
Anthony Mclean, full age, bachelor, inland revenueofficer, West Derby Road, father William Mclean, builder
Amelia Davies, full age, spinster, Newtown, County Montgomery, father , Henry Arthur Davies, (glainer?? possibly from looking at earlier census glazier) both signed and the witnesses were John Lewis and Elizabeth Davies.
The 1851 census has Amelia Davies born in Llanllwchaiarn, Montgomeryshire, Wales and living with her parents Henry Davies aged 33, son in law, b Montgomery, Montgomeryshire, Wales and Elizabeth Davies aged 32, born Newtown, Montgomeryshire, Wales
They are in the home of Amelia's maternal grandparents, William Lewis aged, 57, Grocer and Innkeeper, born Llanidloes, Montgomeryshire, Wales and Elizabeth Lewis aged 56, born Treveglws, Montgomeryshire, Wales
(better stop.. sorry if repeating what you have already found)